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Could a single engine pilot land a passenger jet? (from Airplane)
I'm assuming that almost everyone on this website has seen the movie Airplane at least a few times...
Last time I was watching it I was wondering how plausible the actual act of having a single engine prop pilot, with a lot of training (maybe comparable to a modern day aerobatic pilot), land a quad engine passenger jet?

A passenger jet has several landing aids, talking to ATC who will get a instructor on the phone and will talk him down through the systems.

More likely than someone with no experience at all. Remember, the concept of landing a plane is the same for all planes. The technical systems are different maybe, but the general concept is the same.

We already have this question http://aviation.stackexchange.com/questions/8986/can-a-passenger-realistically-replace-suddenly-incapacitated-pilots

@SentryRaven "Remember, the concept of landing a plane is the same for all planes." That's not really true. The procedures are actually quite different.

Not a dupe -- this is the case of say a PPL with an IR and several hundred to a couple thousand hours TT having to take over, vs. someone with no aeronautical experience whatsoever.  (Or, @voretaq7's flying commercial and both the flight crew conk, now what?)

@UnrecognizedFallingObject the question may be different, but the answer's the same.

@raptortech97 -- I'd say the answer's not quite the same, but that's just me -- hopefully we'll get some more informed opinions on this over here soon!

@reirab Yes, the procedures are a lot different. But the concept is everytime the same: descent, stable approach, flare, touchdown, stop. Whoever learned that flying SEP, will have it in mind with every other airframe.

@SentryRaven The names and basic goals of the phases are the same, but that's about where the similarity ends. I fly SEP, but if I find myself at the controls of an airliner, my most useful knowledge as a pilot will probably be knowing how to declare an emergency and call ATC on the radio asking that they get me a pilot for that type on the radio ASAP to explain how to set up an autoland.

An experienced single engine pilot is with experience "comparable to a modern day aerobatic pilot" is  NOT the "passenger, without any previous flight experience" as written in the "duplicate". Here more maybe a question "how difficult is for a good pilot to fly unknown type of the aircraft"

Yes.
The question will have subjective answers and this is my opinion.  The basics of flying an airplane doesn't change from a single engine airplane to a big airliner.
The concept of lining up on final, flaring into the touchdown attitude and setting it down is exactly the same.  There are some things that are different; for example, retractible landing gear, leading edge slats, spoilers and thrust reversers.
Many of the differences can be overcome by talking with ATC and an instructor / pilot trained to fly that airplane through the radio.  The biggest challenge would be knowing when to flare as you are sitting much higher in an airliner than in a single engine airplane.  This can be overcome with listening to the radar altimeter and flaring as directed by the instructor.

Reverse as a concept might be familiar to a SEP pilot -- I believe the Cessna 206/208/210 have it for instance, although those are generally business/fleet aircraft.  The main thrust of the answer makes sense though -- the basic controls and instruments are standardized enough that a PPL/IR would be able to make some sense of what's going on, and the rest can be handled by someone with a type rating on the other end of the radio.

@UnrecognizedFallingObject Yeah, I'd say getting the "someone with a  type rating on the other end of the radio" is the most important part. Hopefully, they can just talk you through setting up an autoland.

I can relate to you the true story of my uncle who successfully landed a large WWII aircraft with no flight training.
He was the flight surgeon on board a large aircraft that got heavily shot up.  Hostile fire killed or severely injured every crew member.
He moved the pilot out of his seat, and contacted a control tower.  With the expert help of the control tower operators, he successfully landed the aircraft.
After the war, he had a private practice in his home where he saw patients.  For patients too ill to come to him, he performed house calls.  He continued to perform house calls until a short time before he passed.
I have always thought his life story would form a compelling foundation for an inspiring movie.

This does not answer the question as the question asks if a person who has had considerable flight experience on small plane can fly a commercial-type plane in an emergency.

If a person with no flight experience could successfully land a large aircraft with the help of skilled control tower personnel, then it is reasonable to think that someone with considerable small plane flight experience could possibly perform a similar task, given the same type of assistance.  Unless, of course, their experience somehow gets in the way; that's a possibility, but it seems unlikely.

@RockPaperLizard: Especially as the modern aircraft would a) have more aids (such as autopilot/autoland), and b) probably fly better, not being shot full of holes.

@RockPaperLizard WWII-era aircraft are closer to what private pilots fly than they are to the airliners of today. I could probably land, say, a B-17 if I really needed to, probably without even getting help from ATC. Flying a modern jet that flies around Mach 0.9 is quite another story, though.

@jamesqf That assumes you actually know how to use said aids (I'm a pilot and I wouldn't know how to use most of the ones on a modern airliner.) As far as the holes are concerned, unless they've, say, managed to knock off part of the wing or control surfaces or knocked out an engine or two, they wouldn't really affect the flight of most WWII-era planes all that much. Several of those had intentional holes in the fuselage already (such as for guns.) They weren't pressurized back then.

I'd share a personal experience: a few years ago I got the chance to fly a Boeing 737 simulator (the real thing, not the PC simulator!) At that time the only real aircraft I've flown was a Cessna 172. I managed to land on Kai Tak airport in the simulator on my very first attempt to land a (simulated) 737. I was so nervous that I totally forgot to call for the landing checklist, and the instructor helped me with the gear and flaps, but I was the sole manipulator of the controls and I landed with no automation whatsoever, not even ILS.
How? There were a few factors that came into play:

I learnt how planes fly in ground school. Big planes fly just the same way as smaller planes, only with larger wings and more powerful engines.
I knew how to manipulate the flight controls. It came as a bit of a surprise that the controls feel heavier than I'd expect, but they behaved the same anyway.
My enthusiasm in aviation meant I have learnt beyond the Cessna 172 from books, online articles and PC simulation experience. I was not able to identify every single switch and knob in the cockpit, but I knew enough to fly the plane in that situation.

My answer? If it's a perfectly flyable aircraft (no failures), calm weather, then Yes, definitely! Here's what I'd do if it happens in real life:

Aviate, navigate, communicate. No matter what happens, always remember that. That means the first thing to do after entering the cockpit is not to find the radio buttons and call for help; the first thing is to access all primary flight instruments and make sure we're flying.
Radio for help. Declare an emergency on the last radio frequency. If no one responds, tune to 121.5 and try again.
Request a long runway with calm weather. Unless it's something like a A380 or Boeing 747, chances are there would be a runway long enough so you don't have to worry about hitting the touchdown spot.
Setup autoland, if available. Autoland requires suitable equipment both on the plane and on the ground. If it's not available, given a long enough final (say 20 miles), I think I'd feel comfortable hand flying the aircraft down.
Have the emergency trucks standing by. In the event I crashed during landing, gear collapsed or whatever, everyone'd be ready to help.

To step this down a level, I did 30 hours on Cessna 152s at 17, having spent a good few hours on basic (Atari ST!) flight simulators and being keen on the physics and engineering of aircraft. With a few exceptions (odd aero effects like stalling in a steep turn and in particular the ground effect on landing) I felt very at home with the basic flight and controls, in retrospect I'd have given myself a 50/50 chance of an landing without damage to the aircraft if thrown into it from scratch. How this scales up is the question though.

My dad, an experienced pilot, was once given the chance to fly a 727 simulator (full motion, the works). He stalled it 40 feet over the runway and the simulator came down hard enough on the stops to hurt his back and scramble the gyros.
That's only one data point, but any rate, the crash would have been survivable, so there's that.

Like anything in aviation, it depends,
There, to date, have been no documented cases of this happening. There have been a few "talk down landings" in smaller aircraft with low time or inexperienced pilots. Similarly there is at least one documented case of an air force B-1B Pilot assisting in landing an airliner he had no experience flying but you can argue he had plenty of jet time. People have tried it in simulators of course but a sim is a sim. Of course things like cross winds and runway length will play into the success rate of an event like this. I will say that the margin of error on these things is small and on a calm day I'm sure it's possible. I would hesitate to give a hard yes to this question since there are far to many factors to take into account but I do feel under the correct circumstances it could be done.
On a small side note, since this question is about Quad Engine planes (747, A380 etc) considering the distances those planes typically fly it's more than likely that there is one if not two relief crews on the plane so the chances of everyone becoming incapacitated and requiring a passenger step up to the plate is slim to none.

That headline is wrong. The B-1 pilot did not land the airliner. The FO (who was presumably typed on the aircraft in question) landed it while the B-1 pilot presumably did normal PNF tasks. Also, a B-1 pilot will presumably be much more familiar with turbine engines and the level of automation found in a modern airliner than a typical private pilot would be.

You are correct I realized that after adding the video link, ill alter my answer to reflect that he only assisted.


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Why can't you ditch your aircraft in the sea?
Disclosure: I'm just a random guy who knows nothing about aviation. :-)

I read:

A voice in my earphones was shouting: “Ditch the plane! Ditch it in the ocean!”
      It must have come from the tanker skipper or one of the destroyer commanders down below, because every jet pilot knows you can’t ditch a jet and survive. The plane would hit the water at a very high a speed, flip over and sink like a stone and they usually explode on impact.

Okay, but the physics-y side of me wonders why you can't do the following:

Get ready to jump out (open windows, remove all attachments, etc.)
Glide down as close to the surface of the ocean as possible, maintaining some momentum
When you're as low as possible (but still high enough to allow this maneuver), suddenly pull up the nose as high as you can while keeping altitude constant as long as possible

I expect that, if done properly, this could nearly halt horizontal motion of the aircraft; when the momentum is finally lost, I expect the plane would suddenly "drop" in the water from a few feet above, after which you can (hopefully?) jump out of the cockpit immediately.
Now I'm not expecting a 100% survival rate here (I'd almost certainly die trying this), but it certainly seems to potentially beat jumping out of your plane at 15,000 feet and risk getting chopped up by the tail or having your parachute not open (and consequently breaking your spine, etc.).
Where am I going wrong with this? Why is ditching your aircraft in the ocean not an option?
(Does it e.g. depend on the type of the plane?)

Comments are not for extended discussion; this conversation has been moved to chat.

The writer was dramatizing things a bit maybe, it's possible to ditch a jet fighter and survive, however your chances are much better ejecting.
Ditching is an option for any aircraft, with some airplanes ditching is the only option if there's a loss of power over water, for example commercial jets have no mode of egress other than the doors. I fly light single engine aircraft and the majority of the time I have no parachute, if my engine throws a rod over a big body of water that's where I'm going to go, and even if I had a chute I'd probably elect to ditch anyway. It's survivable, and many cases people do. The "Miracle on the Hudson" is a famous example, they even made a movie about it, but there are many others where a reasonable percentage of people on board survived.
You are proposing a low speed ditching in a fighter airplane, which has a number of drawbacks:

Ditching speed: You can't stop an aircraft by raising the nose higher because at some point you will stall the wings, and they will lose lift. A wing requires smooth airflow to create lift, if the airflow separates from the top surface of the wing it will stop flying, a condition known as a wing stall. In a jet that speed will likely be in excess of 130 knots (150 mph), and there's nothing you can do to slow it further than that. At that speed water cannot get out of the way fast enough and is very, very hard, not some sort of pillow which will cushion an impact
Slow speed controllability: Fighter jets are designed for high speed maneuverability, not low speed flight, fly too slow and you can lose directional stability and control
Loss of hydraulics: most fighters have assisted controls which require engine power for the pilot to operate easily, lose power and they lose effectiveness even with auxiliary power being generated
Unknown sea conditions: Sea conditions can have an enormous impact on ditching survivability, the successful ditchings I can think of were in relatively calm conditions where the pilot was able to make a good, even contact with the water. Dig a wing in and the drag will swing the airplane around, most likely breaking it up. It's very hard to gauge sea conditions from any sort of altitude; what looks calm from 5,000ft up could very well be heavy swells once you're close

So what you are suggesting is that a pilot skims the surface of possibly high seas, bleeding off speed in an airplane with progressively degraded control authority - which is also on fire. It's possible, but highly unlikely to work.
The most critical reason the pilot in the tale you linked to had to escape the aircraft was that a fire in the engine would melt the linkages for the rudder and elevator controls in the tail, or cause a catastrophic structural failure long before he could have ditched, causing a total loss of control. There's no chance that would have been survivable.

GdD: "momentum is related to the square of velocity of the aircraft": No, momentum is mass times velocity (p=mv; no squared here). You probably mean kinetic energy. It's one half times mass times velocity squared (E=1/2 mv²).

That is what I meant @Curd, I was oversimplifying things a bit as I didn't want to get too much into physics.

Wouldn't the ground effect allow us to reduce airspeed as well ?

@GdD: Thanks for the update! Now what I'm wondering about is, is what you're saying about aerodynamics actually correct? Specifically, the part where you say "A wing flies because the shape of it creates a pocket of low pressure above it, which requires smooth airflow" seems to be based on the classical (incorrect?) explanation of how wings work. From my understanding, most of the lift of a wing comes from simply a redirection of the horizontal force to vertical (due to collision with the wing). Perhaps I'm wrong though? See here.

@Mehrdad I'm not up to actually explaining the details, but GdD's explanation is correct, to within the limits of what I think GdD even attempted to explain - there are whole books on aerodynamics which devote a significant portion to just how a wing actually creates lift. Air above the wing (where "above" and "below" are problematic terms in their own right!) isn't moving faster - doing so would require impartment of energy toward the aircraft rear on the air by the wing (from where?). The air is however at a lower pressure above than below the wing, which contributes to creating lift.

@MichaelKjörling: Hmm, okay, thanks. +1

@Stanislasdrg, sure, if the sea is calm you could use ground effect to reduce your speed some, maybe 10% as a guess, not much overall. If the sea is rough you could be flying into and out of ground effect, which is dangerous if you get too slow, probably one of the reasons most ditching procedures specify to ditch at landing speed.

@MichaelKjörling, you are entirely right, I did over-simplify the explanation of lift as going into it in detail wasn't really key to my answer.

@GdD Yay, something sticks from my studying aerodynamics! :-)

That decade in university paid off @MichaelKjörling :)

Regardless of how wings create lift, we know that stalling is a factor, which is why it's not possible to bleed off all of one's airspeed and maintain control. We could know nothing about the physics of the situation and still be faced with that certain reality.

Note that that later in the story, a Coast Guard flying boat flies over him but cannot land because "the seas were high".  So ditching into those conditions wouldn't be the best idea.

Additionally: it is very difficult to know where is the ocean. You have no references on the relative height. You cannot predict if you see large waves at heights, or just small waves and you are very near the water. This make more difficult to plan a good stall just above the water. Hudson river landing had not such problem, they had a bridge and coast
(buildings, cars) to evaluate distances.

First, I'm taking this to be about fighter jet aircraft and similar. Airliners, general aviation jet aircraft, and others, are different, and there have been plenty of examples of jet aircraft ditching in water and everyone surviving. US Airways flight 1549 is a relatively recent (January 2009) example where an Airbus A320 jet aircraft landed on water. It's not something you want to do, and it's not something that happens regularly, but given the right conditions it absolutely can be survivable.
Modern fighters are unstable. They won't maintain stable, even descending, flight on their own; rather, they require constant, tiny adjustments to their control surfaces just to maintain level flight, let alone maneuver. This is done because it allows much tighter maneuvering, which is a much-sought quality in many small military aircraft. As long as everything is working correctly the computers handle much of the minutiae of this, so that the pilot can focus on their mission.
If the pilot is going to eject, you want to do that at some altitude, if for no other reason then because the parachute needs time to deploy. You don't want to eject near the ground if you can avoid it, and not all planes that have ejection capability are rated for zero-altitude ejections. Few are rated for "zero/zero" (zero altitude, zero forward speed) ejections, which are basically "sit on the tarmac and pull the lever".
Every aircraft (actually, airfoil) has what is known as its stall speed. This, simplified, is the lowest speed at which controlled forward flight is possible. The exact value for the stall speed varies both with the aircraft and with conditions (temperature, load, control surface settings, ...) but any fixed-wing aircraft needs considerable forward speed for the wings to generate sufficient lift to not stall. Landing an airplane is, very broadly speaking, executing a well-controlled stall. Once the aircraft gets below its stall speed, it becomes basically a large brick, and drops towards the ground. (Things like vectorable thrust and VTOL designs make this somewhat more complicated in reality, but the basic principles are similar.)
It is correct that by pitching up, you are going to lose forward speed (due to drag). The axiom when landing a fixed-wing aircraft goes like power for altitude, pitch for speed, meaning that you adjust your altitude primarily by adjusting power, and your speed primarily by adjusting pitch (nose up or down). (Compare this to movies, where the direction an aircraft is moving in is often controlled almost exclusively by the control column or stick, and seemingly almost never with power.)
Now, let's apply all of this to your third bullet point:

When you're as low as possible (but still high enough to allow this maneuver), suddenly pull up the nose as high as you can while keeping altitude constant as long as possible

I expect that, if done properly, this could nearly halt horizontal motion of the aircraft; when the momentum is finally lost, I expect the plane would suddenly "drop" in the water from a few feet above [...]

Yes, it's probably going to slow down the aircraft's forward speed.
Unfortunately, the aircraft will drop when it slows down below its stall speed, at which time it still has a significant forward speed.
This means that when the aircraft hits the water, it is still moving forward quite fast. Fast enough that the water doesn't have time to move out of the way.
What you are doing then is akin to performing the same maneuver over a similarly rough concrete surface. Remember that by the time you are even considering ditching in the first place, the aircraft is probably damaged or crippled in some pretty fundamental way; otherwise, why not just land normally? Even a very hard landing that ends up breaking the landing gear is far better than a ditching. Unless the water is near perfectly smooth (similar to a runway), when the aircraft hits the water, it is going to come to an abrupt halt, which causes significant forces both on the airframe and its occupants. It's like they say, it's not the fall that kills you, it's the sudden acceleration at the end of the fall. (Conditions may apply.)

[...] it certainly seems to potentially beat jumping out of your plane at 15,000 feet and risk getting chopped up by the tail or having your parachute not open [...]

"Risk getting chopped up by the tail" is why any sane ejection system starts by giving you a significant vertical boost. "Having your parachute not open" is a risk, but that's why parachutes are inspected regularly (they are not set-and-forget items by a long shot!) and it probably still beats crashing in an uncontrollable aircraft.

While I agree with your point on "modern" fighter aircraft, this story relates to an F-8 Crusader in the 1960's. :)

Stall speed on the F-8 is 157 mph

@Tim Thank you for that data point. See also Wikipedia which puts the landing speed at 100 mph.

I ended up asking a separate question about the F-8 as Did the F-8 have a stall speed of 1.6x its landing speed?

As a hang-glider pilot, I can say that's exactly how a good hang-glider landing goes.  We don't land with speed and run it off (at least if we've done it properly anyway).  Instead we fly our approach to landing height, burn off speed as necessary to near-stall, then rapidly rotate the wing.  The wing effectively turns into an instant air-brake, and we land on our feet, stationary.
It's essential to point out that the wing is above stall speed all the way until that final flare.  Inertia is a thing, so if you flare with too much speed on then the glider pitches into the air until it hits stall speed, at which point it comes back down again from a lot higher than you want to be.  This generally results in bent aluminium, and often minor injuries to your "undercarriage" as well.
How is this different from a plane?  Firstly, a pilot cannot do with their wings what we can do with a hang-glider.  The pilot can certainly pull the nose up until the plane stalls, but at low speed they simply can't apply that much pitch-up that quickly.  A hang-glider doesn't flare because of airflow over a control surface, it flares because of a massive weight-shift adjustment.
Secondly, even if they could, the plane wouldn't take it. A hang-glider has a stall speed of maybe 20mph.  A jet will be well north of 100mph.  Trying to apply the same kind of flare at 100mph would simply rip the wings off the aircraft.
Thirdly, consider how long the flare takes.  As soon as you're below stall speed, your wing stops flying and you lose control authority.  On a hang-glider landing at maybe 20mph, the flare takes maybe 3-4s - but we're still stable in the same way as a parachute is stable, because our centre of mass is below the (falling) wing, and we have some limited ability to control the wing with our bodyweight.  On an aircraft landing at something over 100mph, it would take much longer for a flare to burn off all forwards speed without pitching the aircraft into the air, and during this time the plane has no control input whatsoever.  Even a positively-stable aircraft such as a high-wing trainer could be knocked sideways by a gust catching a wing; a negatively-stable aircraft such as a fighter jet will simply drop a wing immediately; your control surfaces are no longer working so there's nothing at all you can do about it; and that's an awfully long time to be close to the ground with no say about what the aircraft is doing.
Some jet display teams do a high-AOA pass as part of the display.  Don't be fooled.  You're a long way away, and that sucker is still moving fast.  It just looks slow compared to the speed it was doing earlier!
Note that with a vectored-thrust aircraft like a Harrier, it is possible to completely halt forward speed, because it has the capability to independently apply thrust in a completely different direction.  Most planes can't do that though.
As an aside, it's also interesting to look at birds from this POV.  Most birds can flare and stop relatively easily, but not all of them. Watch a swan landing.  They don't splash down like that for fun, they do it because they have a high stall speed and too much forward momentum.

"Some jet display teams do a high-AOA pass as part of the display." They also have a fully functional aircraft and plenty of altitude and power to get themselves out of the maneuver before it becomes a critical issue - they're not in a broken craft struggling to stay airborne and/or perform a "successful" landing (where "success" = one you can walk/swim away from).

@FreeMan Agreed - trying to pull stunts with an aircraft that wants to be a brick is not likely to work out well. :/

If you've never seen it, there was a particularly scary video taken of Ethiopian Air 961 crashing in the ocean. The plane had been hijacked and the pilot and copilot were having to fight the drunken hijackers off. Still, it's a harrowing illustration of what can go wrong with landing in the ocean

Leul attempted to land parallel with the waves instead of against the waves in an effort to smooth the landing. Seconds prior to contacting the water the aircraft was banked left some ten degrees; the left engine and wingtip struck the water first. The engine acted as a scoop and struck a coral reef, slowing that side of the aircraft quickly and causing the Boeing 767 to suddenly tilt left. The rest of the aircraft then entered the water unevenly, causing it to break apart. Except for the rear part of the airframe, the broken portions of the fuselage sank rapidly. Many passengers died because they inflated their life jackets in the cabin, causing them to be trapped inside by the rising water.

A ten degree bank on any landing would give me more than pause.

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Have Eddy current brakes ever been used/tested on an aircraft?
Eddy current brakes (wiki) --- a.k.a. magnetic brakes --- get quite some attention from train engineers. They are used on Shinkansens, roller coasters, and even (as far as I know) metro trains on Lane C in Prague. They basically consist of a magnet and a rotating disk, where these two never touch. The kinetic energy is converted to heat, as in conventional brakes. While Shinkansens give them up, from what I view it is in favour of recuperating brakes and whence it is irrelevant with respect to aviation.
I wonder whether anyone considered their use in aviation. To me it seems that it could be a good option.

I can see a couple of major problems with them. One is that the breaking force is going to be proportional to the size of the coil, thus much more weight. The second is that they won't hold at a stop, so you would still need regular brakes.

@TomMcW In that case they could be used on some wheels only maybe? (That's probably added infrastructure that's not worth it...) I understand it's not "all that easy", I just wonder how it is :-)

Related patent: Electromagnetic landing gear brakes held by Hamilton Sundstrand.

Given all the crazy brake designs used over the years, I'm sure someone has tested them.

Advantages of eddy current brakes are that they don't wear and that, in the linear form, they can distribute the heat along the rail, avoiding local heat buildup anywhere. However, they are rather heavy and for larger braking forces need to be electromagnetic, so they need electric energy. Neither is a problem on a train (train engines have to be heavy to have enough traction), but both are problem on aircraft. And without rail, the second advantage can't even be realized.

You are proposing a solution to a nonexistent problem. Commercial jet planes do not ever run out of runway because their brakes get an insufficient grip on their wheels. A report of a "braking failure" is the result of the wheels getting an insufficient grip on the ground.
Eddy current brakes are invariably much heavier than friction pad brakes of the same gripping power. Airline operators would rather monitor and replace friction pads than spend the fuel to carry magnetic coils about. And some kind of friction brake would still be needed, since grip of the eddy current brake fades to nothing at full stop.
If a new technology were ever needed to bring heavy landing aircraft to a stop, operators might consider using an electromagnet on the plane to create eddy currents in a specially prepared metal runway. I don't think this would actually work but the research would be hilarious to watch.
Other potential braking technologies to be considered might include: a robust scoop on the plane that could be lowered into a trough filled with water, a runway graded steeply uphill at the end, and a truly moby version of the good old U.S. Navy standby, the cable trap.

Eddy current brake is a misnomer: it's a damper, not a brake as in a device that works on friction forces and can completely stop motion. Eddy current dampers do provide deceleration forces, by inducing whirly electromagnetic currents in a rotating wheel - they could provide extra emergency braking power without installing extra brake pads.
However, aircraft wheel brakes are dimensioned for emergency deceleration, can provide this adequately using friction pads, and their main problem is getting rid of the heat that is created. Eddy current dampers create the same amount of heat...

In my opinion , its an amazing idea to down the speed of an ground running aircraft to a big extend using eddy current braking. The main adavantage of this technology is that ,formation of eddycurrent can be done uniformly and hence it wont create irregular braking impact. Rather thinking about stoping the aircraft it is more relavent to think about reducing the speed within a short time period ,so that , shorter runways will be required for massive aircrafts to land.Hence for takeoff purpose only lenghty runways are required. And its will also make the pilots easy to face the impact of overshooting the runway during landing.

As we know that, there are many accidents taking place now a days in the time of landing (overshooting the runway),which makes the entire flight a bad memmory for the passengers. Like every part of flight , safe landing is also very important to ensure that the entire flight is not life challenging.So it is time to think about an effective braking system that makes the aeronautical industry more safe and less challenging like the "EDDY CURRENT BRAKING SYSTEM".

more braking will only lead to skidding and blown tires. The limit for today's aircraft brakes is the friction between wheel and ground. How the wheel is braked is less important and disk brakes are the most weight efficient solution. Also, it would be more helpful if you offer less opinion and more facts.

When you quote, link (or cite if you only have paper form) the source.

@PeterKämpf: I thought the chief challenge for brakes were aborted take-offs due to the high weight. And it appears in those cases fire is a major problem (as you're converting all the kinetic energy into heat). Not that eddy current brakes would solve this, that's just another means of turning motion into heat.

@MSalters: You're right, I was only thinking of regular landings where too much braking will destroy the tires. Same goes for aborted take-offs, too.

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What are the terrain-following radar modes of the B1-B?
The Rockwell B-1  Lancer is a supersonic-capable variable-sweep wing large bomber operated by the US Air Force. One of its "key features" is terrain-following radar and a fast, heavily-loaded low-altitude mission profile.
Wikipedia has a beautiful high-resolution picture of the inside of its cockpit (scaled down for SO):

Most of these controls are either items I am familiar with or are fairly self explanatory. I have a few questions about the terrain following radar, however, the controls for which are visible in the picture (and the US military is usually very good at blocking things out if they should not be visible in the picture):

This panel shows "TER FLW", which I presume means "Terrain following" and "TER AVD" ("Terrain avoidance"), both in reference to the radar. Clockwise, from left to right I infer that the switches are on/off, mode selector, priority "terrain following or map", rain rejection, terrain avoidance, "clear plane" [?] so many ft above the terrain, range (in nautical miles?), rain rejection, test button, mode selector, and "Ride", which has options of "SMH".
What do all of these mean? Is my inference correct? In particular:

Does Ride S / M / H, mean "Smooth / medium / hard", or how rapidly the plane should adjust its trajectory and thus closely follow the ground? [Hard implying the greatest degree of change]?

What do the letters A to K actually do on the mode selector dial?

Have I correctly identified everything above?

A bonus question would be "Why are there a pair of green feet" just to the left of this panel?

S/M/H = Soft, Medium, Hard. I had an F-111E pilot describe the TFR on H(ard) as being in a Jeep at 500mph.
The A-K modes would seem to be these:

The B-1B has a single Westinghouse APQ-164 radar with a single
antenna. It was developed from the APG-66 used in the F-16. It has a
fixed phased array which is mounted at an angle to reflect enemy radar
emissions downward. The radar can operate in any of eleven modes:

Real-beam ground mapping mode
High-resolution ground mapping mode
Velocity update mode
Ground Map Beacon mode
Terrain Following mode
Terrain Avoidance mode
Precision Position Update mode
High-Altitude Calibration mode
Rendezvous Beacon mode
Rendezvous mode
Weather detection mode

http://www.joebaugher.com/usaf_bombers/newb1_2.html

Looks like you nailed it – and that site is a (very!) interesting story of its development. Do you know what the green feet are?
Re Hard ride: "Naturally, the system was deliberately designed to err on the side of caution, but it would sometimes happen that an error signal would be sent that would cause the aircraft to suddenly zoom upwards at 2.4g for 10 seconds for no reason. On some missions, this would happen as often as once every three minutes." -- yikes!

The A-K is the setting for altitude above the terrain for terrain following mode. Terrain avoidance on the right side shows altitude, but they are letter coded for terrain following because probably is or was classified. But when you would set one of the letter positions, the corresponding TF altitude would show on the TF display screen in front of the pilot. I know what the lowest one is, but won't say it here.

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text:
When does a 'flaps/slats misconfiguration' warning sound during takeoff roll?
If a flight crew begins their takeoff roll and don't have slats and flaps configured properly, when does the system know when to sound the alarm? If it actually sounds during the roll itself, that seems a bit too late, as there then isn't enough time to deploy those surfaces. And it doesn't seem likely that the crew would have the time to look up a new V2 to figure out at what speed they can take off without flaps and slats.

For the Boeing 727-100 and 747-100/200 the answers are:

when does the system know when to sound the alarm?

When the thrust levers are advanced to begin the takeoff, if flaps and slats and a few other things are not in takeoff position, a loud horn sounds.

figure out at what speed they can take off without flaps and slats.

Takeoff is not permitted without flaps and slats. You have to have them. Depending on how many were not in the takeoff position, the takeoff speed would be inordinately high, require runway lengths not available, and might be above the maximum tire speed. In short, as I said, you have to have them.
On the aircraft I mentioned, if you're starting the takeoff roll from a stationary position and advance the thrust levers at the usual rate, you'll get the horn before your ground speed is little more than walking speed, if that. The thrust lever position, while above anything you would use for taxi, is not anywhere near what you could take off with. It's not far along the travel of the thrust levers from idle to full throw.
The procedure taught for a takeoff warning horn at the carriers I flew for was to bring the thrust levers all the way back to idle, brake to a stop, and request clearance from the tower to vacate the runway. Once off the runway, brake to a stop and troubleshoot the problem. In no case were you to troubleshoot the problem while moving on the runway.
In practice, if there was no traffic, once stopped some would stay on the runway to troubleshoot after getting tower clearance to do so. The assumption was that the problem would be quickly corrected and takeoff resumed.
Troubleshooting was a simple matter of checking that everything that was supposed to be done had been done. For the 747, the items that would trigger the takeoff warning horn were:

leading edge flaps not extended, there were lights on the f.e. panel that showed their individual positions,
stabilizer not in the green band, there was an indicator by the trim wheel on the center console.
flaps not in 10 or 20 degrees position
body gear not centered, lights on the f.e. panel showed its position.
speed brake lever not in the down detent.

Recalling these items from memory was a standard question on oral exams.
If everything had been done that was supposed to be and you still got the horn, it was usual to recycle everything. If that didn't fix the problem, which if the problem was the warning system itself would be the case, it was back to the gate for maintenance.

If you missed one configuration step, you might have missed other things and it would seem advisable to take your time to ensure everything was right.

Can you ferry an aircraft with flap/slat problems?

@Harper I don't have a definitive answer for that. My guess would be no since they're basic to aircraft control. I Googled "747-200 MMEL" (master minimum equipment list). From what I quickly read it seems that for slats the answer is no, but maybe for flaps. For example, let's say you couldn't get the flaps down to 30 degrees, but could to 20. The MMEL isn't totally the law as individual carriers develop their own, and there's always negotiation in extreme situations.

"For the Boeing 727-100 and 747-100/200 the answers are: [...]  When the thrust levers are advanced to begin the takeoff, if flaps and slats and a few other things are not in takeoff position, a loud horn sounds."  Also the case for the DC-9-80, according to the NTSB.

On most aircraft I have flown, the Takeoff Config warning sounds when the thrust levers are advanced for takeoff power.

The configuration horn goes off as soon as the aircraft decides the crew is trying to take off instead of just taxi - it's based on the throttle setting.  But the rest of your question has an unspoken inference that continuing a takeoff beyond the point where a configuration warning activates is even conceivable in a professional environment.  The correct crew response would be an immediate abort and identification of the problem once clear of the runway.  Anything else would be unacceptably risky and not justifiable under any circumstances.

I was kind of thinking of what the flight crew of Spanair 5022 could have done to had their configuration horn been working or had they realized at some point during their takeoff roll that they had not deployed flaps and slats. I tried to phrase it in a more general tone.

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Do military bases still employ wheel watchers?
When I was in the air wing in the '70s, I had the unpleasant opportunity of pulling wheel watch. The unfortunate individual sat in a small shack (maybe 3 x 6 ft.) with a pair of high-powered binoculars ensuring that each and every aircraft on final had their gears down. If not, there were two flare guns pointed at opposite right angles to the runway that had to be fired to warn the pilot off. I know I had to use them on more than one occasion. I think it was a thirty-day hitch. It seems like such a primitive way to ensure the gears were down, and I wonder if it's still being used today?

Just out of curiosity, do you remember what the criteria was for firing the flares?

@Terry, simply the inability to see the gears. In the smog of the Los Angeles area in midsummer, it was sometimes difficult to see, so I waved off some planes simply because I couldn't tell if they were down or not.

Wheels up landings must be a common problem in the military. ATC always clears military aircraft to land adding, "confirm wheels down."

Civilian ATC used to say check gear down but stopped some years ago.  When I was flying RJs the gear warning horn was a steady double freq tone that sounds exactly like the old TV test pattern sound of long ago.  I always thought it was perfect for blending into the background in a high stress situation where the gear was omitted.  Fortunately, if that happens the "Too low, gear" GPWS announcement comes farther down to save your butt.. hopefully.

My favorite time was on wheel watch. I got to put a flare into the T-28's prop with a solo student pilot (1965).

@TomMcW That might just be because military ATC does that and civilian ATC wants to be consistent in case some pilots have become reliant on the reminder.

@DavidRicherby I'm sure that's what it is. They're prob required to do that for all military. I'm not sure, though, if military controllers give that warning to civilian flights or not. Never listened to military ATC. They prob do out of habit, though.

Not an answer, so putting as a comment - I would guess that most modern commercial aircraft have some sort of proximity alarm if the gear is not down too close to the ground, giving an automated warning.  Do military aircraft have similar warnings?  If not, that may be the reason that they (at least used to) provide external wheel watches to provide a secondary gear check that commercial aircraft have built into their warning systems?

@Milwrdfan Yes, they do. In the military, the audio warning system is lovingly called "Bitchin' Betty", but it will give warnings for low altitude with gear down, terrain collision/proximity, etc. There's similar warning systems for commercial aircraft, and you install them in GA aircraft if you're so inclined.

@JohnK - reminds me of a story where a F-4 pilot was flying along when an Atoll went past the canopy. He never heard the RWR. But then they played the tapes and he also didn't hear the ATC and RIO both screaming at him!

@MauryMarkowitz  I've had the experience, fortunately only in the simulator, of being mentally saturated, where at the end of a 4 hour session we did a wind shear escape maneuver and I remember being mentally fried and just sitting in the climb following the escape from the virtual downburst, gazing at the attitude display, until my partner nudged me to level off.  A Brit Air CRJ200 crew landed wheels up after a flap fail at 0 degrees (where you land at 160kt) and missed both the gear horn and the GPWS.

@TomMcW the way the .65 is written, all military controllers (and civilian controllers working at military airports) must issue the "check wheels down" to all aircraft they talk to. But a civilian controller working at a non-military airport doesn't have to issue "check wheels down" to a military aircraft.

No
They do not continue this practice anymore. This is likely because landing gear are more reliable now than they were in the 70's. There is also the practice of Air Traffic Control prompting military pilots to check their gear is down, by verbally adding "Check Gear Down" to their landing clearance.
Source: Checked in with some of the Air Force / Air National Guard pilots in the office. They're not aware of any such practice and decided it must have been before their time. Oldest pilot started in the 90's, so that leaves you with a 20 year gap where they abolished the practice.

I regularly fly at a dual-use field and I don't know how many times I got told "check gear down" while I was in a 172 before I decided that it was habit, not the controller mocking my tiny little GA plane.

@ErinAnne  I think the reply to that, "roger, gear down & welded" has been used a few times to remind the controller that his query is really only appropriate for retractable-gear aircraft.  Believe that this distinction was/is in their regs, although of course you're right that habit patterns are hard to overcome at times.

@RalphJ Do you think that controllers have to be aware about exact specifications of your aircraft? Moreover, there are some 172RG in the air.

...why wouldn't military pilots put their gear down? Why do they need to be reminded?

@BruceWayne That's pretty much a microquestion, but to answer it simply: Military aircraft are expensive (90,000,000 USD each for some). In the pursuit of not destroying a 90M USD aircraft, they add the extra two seconds per flight to confirm gear are down. It's a simple fix to a problem that only needs to happen once to be devastating.

@Matt or to put it another way, add up two seconds of the controllers time per flight and see how long it takes to hit $90M in added costs. (I get something like 2.75 billion landings)

@Michael Well, military controllers generally don't operate at verbal capacity, there' usually at least a few seconds of downtime between traffic calls, so I don't think there's any lost time, plus there needs to be safe separation - the controllers don't get paid per word spoken, they get paid per day. So, it's actually zero additional cost, save for the cost of the added ink in the controllers manual!

@BruceWayne When working in "never fail" environments, you want to design systems so that all observed failures are at least one (or more) safety step away from disaster.  If 99.99999% of the time people don't need to be reminded, that means 0.00001% of the time they do.  At 100 million per failure, that is a average cost of 100$ for failing to remind someone to check landing gear down.  So you say "check gear down" every time, even though over a career of 100 flights/day, 200 days/year, 40 years you won't typically have even one time it matters.

I doubt even the most minimally competent pilot would ever need to be reminded to extend their gear.  It's more a matter of how much you trust that the gear are indeed doing what you told them to do and there is not some sort of mechanical failure (which somehow managed to avoid setting off any warning indicators).  It's a check on the mechanics of the plane, not a reminder for the pilot.

@DarrelHoffman Ha - you need to meet more pilots. After a mission that lasts half a day, I'm surprised they remember to land at the right base... and sometimes they don't even land at the right base either! They are all still human.

@Matt But there are plenty of other stupid mistakes that pilots can do to destroy their expensive planes.  Why should ATC care about leaving the gear up, specifically?  For instance, before takeoff, they don't remind pilots to check their controls are free and correct, or that they have fuel.

@Maxpm because in this case it isn't insurance that is paying for the plane but the brass up top that then needs to cut into their budget to repair/replace the ruined plane. Some bean counter calculated that reminding the pilots saved enough planes that the manpower cost is worth it.

@Maxpm On the ground, they have crew that doublechecks fuel. Controls, that seems a bit unnecessary. There's a lengthy startup procedure and preflight checklist, which is pretty easy to follow when parked on the ground, but a bit harder to follow checklists to the dot when you're flying a highspeed fighter.

I think sheer cost of the jets is a red herring, at least partially. 787's also run 100+ million, and ATC doesn't warn them. Clearly some combination of risk/benefit, tradition, (over?)caution, and random whims of management play into the military and civilian ATC's decision, but good luck to anyone trying to disentangle them...

@mbrig Not quite. Commercial operators have insurance, military operators do not. 90M is a ton of money, and the military can't afford to replace jets while also funding the rest of their projects. Their budget does not exceed their wants.

@Matt Insurance trades irregular costs for fixed ones, but doesn't decrease the costs themselves (in fact it increases them). If not for the sake of money, I would at least expect the risk to human lives (of which there tend to be more on civilian planes) would have airlines (or their insurance companies) having this reminder added. Given that they don't, I expect there's some kind of other factor. Maybe historically, military landing gears had a lot of reliability problems and the tradition carried forward? But I don't buy a cost argument.

@mbrig Landing with gear up doesn't always cause loss of life (I saw it happen a couple times during my private pilot training with GA airplanes, no injuries). It is definitely (primarily) a cost argument, as it's such a minor fix for such a major potential cost. The military has limited aircraft as well (many production lines are closed for certain models), so they can't just go out and buy a new one.

Remember that most fighters can carry external stores from fuel to munitions. Scraping a center-line fuel tank along the runway instead of the belly of the plane, or having wing mounted bombs or rockets creating lots o' sparks can lead to a much bigger boom than just the plane itself. It's (one of) the same reason(s) that a commercial jet liner that just took off and developed a mechanical problem will circle or dump fuel - to minimize the potential danger on landing.

In the early 1990s the Navy used to deploy instructor and student pilots to perform wheels watch at outlying fields that were NOT tower controlled, and where student solo pilots would practice landings during primary flight training in the T-34C.  Without an instructor or tower this practice made sense.  I cannot speak to whether or not it is on-going.
Once complete with primary training, any landing practice at non-tower controlled outlying fields would be done with a carrier Landing Signals Officer on station.  The LSO would note trends, grade all landings and also function as a wheels watch, although wheels watch wasn't the primary purpose.
During 20 years spanning the mid 1980s to early 2000s I was never aware of any wheels watch at a tower controlled military airfield, but that's not to say it didn't happen somewhere.

"but that's not to say it didn't happen somewhere."
I would think otherwise, as a pilot would no doubt be told what to do if he saw two flares go off at the end of the runway - meaning he would know that there is the potential for wheel watchers. If a pilot was never instructed about this scenario, it would almost certainly mean that wheel watchers were not in effect. Cool stuff about the LSO though! And makes sense that they would have someone watching for student pilots.

This was MCAS El Toro, which at the time was the headquarters for the 3rd Marin Air Wing. I was with H&HS repairing TACANs and transponders. We most definitely had a tower.

Good point @Matt.  "What does it mean if you see two flares?" was never considered or briefed in all my time after leaving the training environment.  In fact I don't even specifically remember discussing flares in training.  The time I stood wheels watch with my instructor I'm pretty sure we had a hand held radio, although there may have been a flare gun in the kit as back up.  It was a long time ago now!

@MichaelHall "red pyrotechnics" is still listed in civilian Canadian pilot training material as an indication not to land at a military field (or it was last time I saw them...). Which isn't to say that they still use them.

@MichaelHall - I'd imagine, that even without a briefing, human instinct would kick in if you saw flares going off across the runway. At minimum you'd radio in to find out what's going on, if not initiate a go around automatically.

@FreeMan, I was going too respond saying the exact same thing!

I remember the AF still had the runway supervisory unit (RSU) when I was still on active duty in the '89-'92 time frame. I wasn't an AF pilot, but I was a private pilot flying with the Aero Club at the time. I'm pretty sure the RSU wasn't staffed full time. It may have been limited to periods of high op tempo when the consequences of a gear up landing would be more severe. I don't remember seeing an RSU in the years since then.
Edit:  I was reminded that the field got a new control tower shortly after I left. I believe the RSU was removed at that time as it was no longer needed. The new tower provided much better visibility to that runway end than the old tower.
We did have the 'check gear down' calls from the tower in those days. I think the RSU became redundant to the ATC call/Pilot confirmation. For a/c with 2 crew, you had a backup check on board. Most of the fighters by that time had the landing lights on the gear, so in addition to the ATC call, the tower could just look for the landing lights.
As for the reason for the calls, it's not just about the potential loss of the a/c. In a combat environment, the runway is a critical asset. A gear up landing would render the runway unusable for a time preventing other a/c from landing or taking off. That can seriously impact the mission. Similarly, the Navy has been known to push an a/c overboard to clear the deck of a carrier.
The second factor is the culture. In the civil world ATC has their job and pilots have theirs. They don't want to cross that boundary. In the military, everyone is part of the team and they work together to help ensure mission success.

"A gear up landing would render the runway unusable for a time preventing other a/c from landing or taking off" - not necessarily. A few years back I was watching a joint European airforces exercise at a UK airbase. Somebody had a problem (not a wheels-up landing) which blocked the only runway. Big deal - the next few groups to depart just did formation takeoffs from one of the taxiways until the tow truck had removed the blockage!

As a current student naval aviator, I can tell you that Wheels Watch (who is a student assisting the Runway Duty Officer) is something that is still alive and well for the Navy Outlying Fields that we train at in addition to home field (North Whiting KNSE), which has a controlling tower.
To speak to the operational necessity of this safety measure, T-6B's still manage to have gear up landings at OLFs, so when you have an excess of warm bodies to throw at a problem you are inclined to keep using them.

Nice info. MCAS El Toro wasn't exactly an outlying field, and the reference to "wheel watch" meant sitting in that small hut ensuring each and every plane making a landing had their gears down.

Welcome to [aviation.se], thanks for your contribution, and thank you very much for your service!

Last time I was in the RDO shack (when solos were flying) was at NAS Corpus Christi about 15 years ago.  Thanks for the update.  I guess some things don't change. Fly safe.


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If a pilot's plane suddenly and unintentionally becomes inverted, what is the pilot's best course of action?
If a plane was suddenly inverted through no action of the pilot, say wake turbulence from a much large plane (think 747 vs. Learjet), should the pilot first try to right the plane or get the nose pointed in the correct position so that the wings start producing lift again?I would think that a pilot would instinctively try to get the plane righted, but I wonder if this would be the best course of action, especially if there's not a lot of space between the plane and the ground. In trying to do so, the pilot could lose precious seconds that would otherwise be spent avoiding the ground. Though possibly counterintuitive, it seems like it would be best to get the plane flying, even though it's inverted, and then get it right side up.This brings up another question: are pilots trained how to handle an inverted aircraft? But I think it's a separate question.

The scenario described in this question would not happen. In reality, if the turbulence is that strong, the plane will break apart.

@kevin that's not entirely accurate.

@kevin, I have heard at least one reliable account of an airliner (B737) rolling past 90° due to wake vortices. The wake is not usually violent to the airframe, more often it is improper recovery technique that overstresses the airframe.

@kevin, wake turbulence from a Boeing 767 also brought down a Learjet 45 landing in Mexico City that was carrying Mexico's (I believe) Interior Secretary. The Learjet crashed in downtown Mexico City. Sorry, I don't recall the year, but can probably find it if you're interested.

Just looked it up; it was in 2008.

@BillDOe That's XC-VMC you are thinking of.

What you are describing is Unusual Attitude Recovery (UAR) techniques, which are a standard part of the training for every Private Pilot1
Here is a good description of just such an event.
Almost all pilots do instinctively want to right the plane first, but as I'll explain below, this is a mistake, and the point of UAR training is to change the pilot's instinctive approach.
The actual biggest threat to a plane in an unexpected unusual attitude is speed.   If the plane has too much speed (above VA), sudden control movements can damage the airframe or even break apart the plane.
If the plane has too little speed (stalled or spinning), there may not be effective airflow over the wings and tail, and moving the controls may not do anything at all! (remember, control surfaces need a steady flow of air over them to be effective).
So the very first and most important step in UAR is to manage speed.  Upon recognizing that they are in an unknown and unusual attitude, the pilot's immediate action must be to look at the airspeed indicator:  If its too high (yellow-arc or above), pull the throttle to idle and let the speed come down.  If its too low (white-arc or below), immediately add full throttle and let the speed come back up.
Once the airspeed is anywhere in the green-arc (and precision is not terribly important here, just a manageable speed), then the next steps are to return the plane to roughly level (+/- 10° of pitch), and roll the wings level.

1 I'm unsure if Recreational Pilots and Sport Pilots also get UAR training.

A minor detail, but in your last paragraph the final steps differ depending on the type of upset: If you’re “nose high” and banked, you should pitch down first, then roll level; however if you’re “nose low” and banked, you should roll level first before carefully pitching up (otherwise you might pull into an ever tighter spiral, overstressing the airframe).

For a low nose, high bank upset, you would not wait after "reducing throttle" for the speed to come down before levelling the wings. For a sufficiently high bank angle/low nose (especially on jet aircraft), only reducing the throttle  will not be sufficient to even decelerate the aircraft (especially not if past 90 degrees bank). You would simultaneously reduce throttle and level wings. When wings are near level, you would smoothly raise the nose. Technique differs by acft. With low mounted engines, full pwr at high pitch/low speed can result in a pitch up moment you cant overcome by elevator.

Check out FSF Airplane Upset Recovery training aid: https://flightsafety.org/wp-content/uploads/2016/09/AP_UpsetRecovery_Book.pdf with section 2.6.3 containing guidance on actual techniques for recovery


text:
Do airliners adjust altitude for latitude?
According to this NASA article, the troposphere extends from about 5 miles at the poles to about 9 miles at the equator. Though the height of the troposphere is defined by the height of the tropopause as per this Aviation SE question.
I'd expect that the air pressure at the top of the troposphere would be similar at 5 miles above the poles as it would be at 9 miles above the equator.
As the altitude of an airliner increases, the angle of attack necessary to produce the same amount of lift also increases. Eventually an altitude is reached where the angle of attack necessary to produce sufficient lift is also the stall angle of attack. This pretty much defines an airplane's service ceiling. (Certainly, there are other factors, but this is a pretty big one.)
This suggests that the service ceiling would be lower at the poles than at the equator. Does this mean that an airliner flying a polar route would fly at a lower altitude than one staying in the temperate or tropical regions?

Aircraft at high altitudes fly based on pressure altitude. Also, pressure/temperature at the tropopause is not the same at the equator and the poles.

You are confusing altitude with height

This figure, which its isobars used for flying at high altitude illustrates both @J.Hougaard 's idea and the answer to your question. It's part of the Wikipedia article on altitude.

Does this mean that an airliner flying a polar route would fly at a lower altitude than one staying in the temperate or tropical regions?

Yes but not "this".
Your understanding of the effects of altitude is correct, except

It is air density rather than air pressure that plays major role aerodynamically. Density falls with altitude similarly to pressure but a bit slower.
Airplanes can compensate lower density with higher speed to avoid increasing the angle of attack, until they hit another limit.

Generally, distribution of pressure and density with altitude does not depend significantly on latitude, especially if we use (as we should) the correct geopotential altitude. As the diagram in the ymb1's answer shows, pressure depends much more on weather than on latitude.
At the same time, tropopause is defined exclusively by temperature. Unlike pressure and density, which have smooth distribution, it has "special points" where distribution "breaks". One of such points defines the troposphere/tropopause border. And temperature does depend on latitude.
Nevertheless, if you are flying a polar route, indeed you'll want to fly lower. But this is simply because flying higher when temperature doesn't fall anymore doesn't make much sense, primarily because engines efficiency won't increase. You may want to climb for other reasons though, but that's another topic.


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How to calculate the lift starting from the vertical speed?
Is it possible to calculate the lift of an aircraft knowing its speed (IAS,TAS and GS), its vertical speed, its altitude and its weight?
Because if we look at the force of an aircraft in flight: the lift is opposed to the weight and if the lift is greater than the weight, the aircraft climbs with a certain rate (vertical speed) proportional to the lift.

It could be worth mentioning that the question is related to wings-level flight (e.g. no bank).

Yes, it is, at least for small pitch angles and low thrust. Here is how:
$$L = m\cdot g\cdot n_z\cdot\cos\gamma$$
If thrust or pitch attitude become significant, you need to modify $m\cdot g$ by the vertical thrust component. Then the answer must be no, because the thrust information is missing:
$$L = \left(m\cdot g - T\cdot\sin(\Theta + \sigma)\right)\cdot n_z\cdot\cos\gamma$$
$L\:\:\:$  lift
$m\:\:$ mass
$g\:\:\:$ gravitational acceleration
$n_z\:\:$ load factor (= 1 in horizontal, unaccelerated flight)
$\gamma\:\:\:$ flight path angle, with $\gamma = asin\left(\frac{v_z}{v}\right)$
$v_z\:\:$ vertical speed
$v\:\:\:$ flight speed (TAS)
$T\:\:$ Thrust
$\Theta\:\:$ pitch angle (up is positive)
$\sigma\:\:$ thrust angle relative to airframe's x axis
Note that the altitude is only needed if you use the altitude-specific value of $g$. Using the standard value will give you a negligible error.
Please note also that lift > weight will result in an upward acceleration. In a quasi-stationary climb or sink the lift will be slightly smaller than the weight.

What is the flight speed? TAS/IAS/GS?

@jd943bd: TAS, and make sure you use the same units for flight and vertical speed.

Does this assume that pitch is the same as the flight path? It seems like it's common for pilots to climb by increasing thrust without changing pitch

@raptortech97, pitch is flight path plus angle of attack, always. This assumes angle of attack is small.

@raptortech97: Pilots increase thrust and keep trim constant when they want to climb. The pitch angle is selected by the aircraft, and of course it increases in a climb.


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How to read performance chart?
From the Flight Planning and Performance quiz for PPL, there is a question asking to calculate the distance needed for landing over 50ft barrier.
According to the example shown by the manufacturer (dashed arrows), once the wind line is intercepted, the distance needed is calculated by reading the value of the horizontal axis of the point.
According to the quiz answer (red line), the distance is calculated by following the slope of the parallel lines and reading the point intercepted on the horizontal axis.
The question is: is it correct to go straight down or to follow the shape of the parallel lines?

@757toga I agree manufacturer is probably more reliable, but this question gave me doubts! The question has been extracted from PPLTutor

Someone at PPLTutor made a boo boo.  If you were supposed to follow the slope of the vertical landing distance lines the way the red vertical line does, you might as well not slope them.
It's the slope of the "ground roll/50 ft barrier" lines (one each for headwind, no-wind, and tailwind) that adjusts the landing distance for a given altitude and temperature, and for the slope to be meaningful, you have to be projecting straight down from the intersection point of the horizontal projection line, so that the higher and hotter you are, the landing distance increases as the slope of the line carries your vertical distance projection line to the right.


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What is the relation between the Lift Coefficient and the Angle of Attack?
I'm trying to understand the relation between the Lift Coefficient and the Angle of Attack using PEP software for the A320 IAE. However, I notice that often for a given Angle of Attack, there could be more than one Lift Coefficient ("slightly different" but different).
Is it an error or a real issue? If yes, what are the standard conditions assumed for the Cl vs AOA graph?

The airfoil of most aircraft is not identical along the wing, so naturally there would be a difference in Lift coefficient at different points along the wing.

What do you mean by 'slightly different'? Like a 0.001 difference, or a difference of 0.1?

0.1/0.2 of difference

Normally the relation is linear within the practical range of angles of attack used in flight. However, the Mach number and the Reynolds number will cause shifts. For the Mach effect, use the Prandtl-Glauert rule in subsonic flow up to Mach 0.7: $$(\alpha - \alpha_0)_{Mach} = (\alpha - \alpha_0)_{incompressible}\cdot\sqrt{1-Mach^2}$$
A higher Reynolds number means a relatively thinner boundary layer, and since the boundary layer on the suction side of the airfoil grows in thickness with increasing angle of attack, the same airfoil will have less boundary layer growth at a higher Reynolds number. This translates into a higher effective angle of attack at the same geometrical angle, so the lift curve slope becomes steeper and will reach stall at a higher angle.
There are no standard conditions, but a proper lift curve graph should note both the Mach and Reynolds number for which it is valid.

And what happens in a plot where I have to show the full range of Angle of Attacks where there is the stall: the speed changes for sure, thus the graph will be inconsistent, isn't it?

@jd943bd: For a real aircraft plot you're right. Normally, the polar plot is done for one Mach and Re, and you would need several of them to cover the flight regime. If you plot the actual lift coefficient at the actual Mach and Re, the biggest changes are at small angles of attack. Around stall the change is negligible.


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Why is the flow ahead of a shock wave chaotic?
Here is the image of a shock wave on a cone in a small supersonic wind tunnel, the white lines are the condensations formed in the test chamber due to flow expansion.

Why are the condensations so chaotic ahead of the shock wave?
I know that the flow shouldn't fall down on the cone but still, why is it so chaotic?
Note: the chamber is 4-5 cm long.

The wind tunnel design is not providing clean smooth airflow. Turbulence at supersonic speed does not travel upstream so the turbulence must be created by the wind tunnel design.

Why not? IS the wind tunnel poorly designed?

How would I know why?  The fact is turbulence at supersonic speed does not travel upstream so the turbulence must be created by the wind tunnel design. The picture has no detail of the wind tunnel and even if it included the whole tunnel the small details in wind-tunnels can have very large effect.

turbulence at supersonic speed does not travel upstream so the turbulence must be created by the wind tunnel design would be a good addition to the answer.


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Dead reckoning navigation: gyromagnetic compass, how it is stabilized?
I am studying aircraft navigation systems, as what concern autonomous navigation a compass is obviously essential, but magnetic compass is not reliable in short term maneuvers due to accelerations so it has to be stabilized by a directional gyro. What i don't understand is how the gyro is connected to the compass and stabilizes it.
In modern airliners is this device still used ? Is there an electronic substitute with no mechanical gyro (a part from GPS system) ?

Gyromagnetic compass seems to be a contradiction, either the compass is a magnet, or a gyroscope with Schuler tuning.

Gyromagnetic compasses do indeed exist even in smaller aircraft, and need not be overly complex. The basic principle for an analog system is as follows:
Heading is measured against a gyro rotating in the vertical plane (just like a normal heading indicator). The gyro may be in the instrument case, or electronically repeated from elsewhere in the aircraft.
The earth's magnetic field is sensed by a sensing unit, a "Flux valve" usually located in the wing, or otherwise in a good position to minimize electromagnetic interference from the aircraft.
The gyro position and the magnetic reference are compared electronically and the resulting difference becomes an error signal. This error signal is sent to electric motors on the gyro frame, which will move to very slowly (a few degrees per minute) correct the gyro to align with the magnetic reference, combining the short term accuracy of the gyro with the long term accuracy of the earth magnetic field. To further enhance accuracy, cut-out switches cuts the error signal out when the aircraft is banking/accelerating, as this tends to give a less precise magnetic signal from the flux valve, while a gyro is unaffected by it. See the Bendix King KCS55A for a real life example.
On older, larger aircraft, you'll find similar installations. On modern aircraft, with inertial navigation platforms (IRS) heading is often taken from the inertial platform as true heading, with magnetic variation applied from a database to provide magnetic heading.
The IRU will in that case (among other things) act as a kind of gyrocompass, finding true north by leveling and rotating 3 nowadays usually mathematically modelled gyro planes (solid state Ring Laser Gyros, no moving parts), to align with true north, by sensing gravitation and angular movement around gyro axes from the rotation of the earth. The planes are then kept level by integrating gyro data, and filtering/fusing it with accelerometer data, movement over earth curvature, Schuler tuning, other sensor data etc.)

How is the gyroscope connected to the compass? In small aircrafts, they don't. The gyroscope itself has no idea which way is North. There is a knob next to the Direction Indicator which rotates the display. You turn it to synchronize the instrument with your magnetic compass.
As you fly, the gyroscope will drift, so you have to synchronize it once in a while. The magnetic compass is not accurate if the aircraft is turning. Therefore, you should only synchronize when flying straight.
In short:

The direction gyro is accurate during turns, but will slowly drift as time goes by. Use this during maneuvers, which lasts a short period of time.
The magnetic compass is inaccurate during turns, but will not drift.

In this picture, the left knob rotates the instrument. The right knob sets the heading bug.

In a modern airliner, the flight computer integrates electronic gyroscopes (gyroscopes on small aircrafts are often mechanic), electronic accelerometers, magnetic compass and GPS position data to determine the plane's heading, position and orientation. The synchronization is done using a set of (very complicated) math.

Slaved compasses have a flux gate connected to the gyro compass to adjust for magnetic north.

Interesting (-: how long has it been around? Surely my instructor never mentioned it.


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Why are downward pointing winglets more efficient?
While most aircraft have upward-pointing winglets

Source
there are also some UAVs (I couldn't find any manned aircraft$^1$) with downward pointing winglets.

Source
I'm pretty sure that other considerations such as ground clearance, propeller/camera system protection and stability were the determining factors in the examples showed above.
However, according to a study (p.11) referenced in this question downward-pointing winglets are more efficient.

This also means that winglets sticking up are not as efficient as
  folding them down.

I couldn't find the sources mentioned in the paper.
What makes downward pointing winglets more efficient? What are the efficiency-related differences between the two directions?

$^1$: No, neither the A-10's nor similar wingtips count.

For efficiency the orientation of winglets is not important. Best would be to stretch them out horizontally, but on a canard they do double duty as vertical tails.
Things look different when maneuvering is added: Now the downward-pointing winglet is superior especially if it carries a rudder. When roll is commanded by deflecting the ailerons, the lift change on the outer wing will carry over to the winglet. On upward-facing winglets this creates an opposite yawing moment while on downward-facing winglets this yawing moment supports the intended turning maneuver.
Also, they look cooler.

They look very cool indeed. I read somewhere that in aeronautics, if it looks good it usually is.

I'd have though that the direction doesn't matter too (vortex induced vertical speed component on the wing should be equal which implies equal induced drag). After reading the paper again I'm thinking that it probably is a comprehension problem on my side and the folding them down of the paper just means no winglet at all instead of pointing downward. The drag advantage you mention in the question I linked is then only meant with deflected ailerons?

@Gypaets: Yes, only when maneuvering, i.e. with ailerons or rolling motion. And they need to be behind the cg to do that.

While airliner winglets don't carry control surfaces, I'd imagine that the improved maneuverability of a down turned winglet is actually a detriment to an airliner. Fighter jocks and UAV "pilots" love a quick snap roll, but Grandma and Grandad want a nice, Cadillac-smooth ride on their way to Florida or to see the grand kids.

One aspect of efficiency that downward pointing wingtips improve is increased bottom lift from increased compression of air underneath.  "End caps", wing tip fuel tanks, anhedralled wings, Lippisch ears, downward folding wing tips (as seen on the XB-70), and downward pointing winglets achieve the same effect.
These benefits must be weighed against the increased drag (with the exception of the XB-70) adding them to the design produces.
However, for the smaller drone pictured above, the downward pointing wingtips also serve a very important function best described by one of my mentors as follows: "If you were a seagull, you would not want a cross wind blowing you into a cliff."  The downward pointing wingtips roll and yaw the plane into a cross wind gust, whereas the "classic" upward pointed vertical stabilizer design will tend to roll away from the gust, especially if the gust gets "under" the windward wing.  For a very light model (or bird), the result is getting rolled and pushed downwind.  Not a good day for that seagull.


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Is a fixed wing more efficient in reaching a certain altitude than rotary wing?
Given the limited amount of energy (battery or fuel) let's reach the maximum possible altitude!
Rotary wing aircraft pushes the air directly downwards and propels itself straight up.
Fixed wing, however, must gain forward speed to produce lift, so it wastes some energy for unnecessary circling and associated drag. Anyway, eventually, its wings must somehow push the air downwards to gain altitude (that's what Newton says), energy-wise is it really different from rotary wing? (especially when circling really tightly?)
Moreover, imagine our fixed might wants to pitch up really hard and has powerful engine - at some point this makes it similar to rotary wing, in that the thrust direction becomes more and more vertical. Oh, so the distinction might be not that obvious!
Anyway, the question is, for this specific requirement of going just up, is the fixed wing still more energy efficient to reach a certain altitude than a rotary wing and why?

I'm not sure this is even a question, when is the last time you heard of a helicopter flying at 50,000 ft? World record is 42,500...

@RonBeyer I think that "Max Altitude" was just thrown in there because the question at the end asks more of "Is a fixed wing more efficient in getting to a given altitude than a rotary wing?".

@SMSvonderTann Yes, exactly, I meant the cases where the limiting factor is energy, and not the unfavorable physical conditions at high altitudes. Of course this is still connected somehow...

@SMSvonderTann I rolled back your edit because it significantly changes the question, fuel efficiency can't be used to compare rotary/fixed wing in this context.

Please note that helicopters will climb faster while moving forward. To gain altitude pilots will sometimes fly a climbing spiral

This question is too broad.  Is the "time to altitude" compared from a dead stop for both, or for both already flying?  What is the start point, what is the end point?  I am familiar with a comparison between a CH-53E and a Jet (IIRC, TA-4) done back in the early 1980's but that was based on engines turning, near sea level, warm day, SFC to 10,000.  By the way, your question is wrong. Rotary wing aircraft pushes the air directly downwards and propels itself straight up. Nope, that is incorrect.

If your (long) question boils down to "do we need more energy to climb to a certain altitude following a spiral rather than a vertical, all other factors unchanged", then the answer is yes. Just imagine this is not a spiral, but a constant climb in straight line: You can do the same in two steps, first climb vertical, then move horizontally. The second part needs additional energy by itself. But sometimes it is necessary to use a slope rather than a vertical due to the maximum work that can be done by the engine per second (other name for power).

A helicopter in forward flight is more efficient than a helicopter in vertical hover, due to less induced drag (and more form drag which becomes a problem at high fwd speed).
Fixed wing aircraft are more efficient than forward flying helicopters because:

The rotor flow in forward flight causes fearsomely complex aerodynamic interactions in flow with the fuselage and other components, which causes a particular type of drag that a fixed wing simply does not have.
When the helicopter is in forward flight, the rotor disk has the same characteristics as a fixed wing: less induced drag, higher form drag. But it will be a circular wing, which is always less efficient than a beautiful slender fixed wing. The slenderer (is that a word?) the better.

The first human powered airplane flew in 1977, and a later version https://en.wikipedia.org/wiki/MacCready_Gossamer_Albatross in 1979.
The first human powered helicopter flew in 1989.  The Wikipedia article mentions several endurance records (measured in minutes), but no distance records.
It stands to reason that if a rotary wing were more efficient, human powered helicopters would have been built prior to human powered fixed wing aircraft and would have accomplished more.

The first human powered helicopter flew in 1989.

@RonBeyer - thanks, I've updated the answer.

It is worth noting that on a fixed wing aircraft the aerofoil shape of the wing creates low and high pressure rather than, as you said "its wings must somehow push the air downwards to gain altitude"
Technically speaking the Rotors on Helicopters are much the same (aerofoil) shape as wings on a fixed wing aircraft. The difference is they're forced to move through the air at speed by the engines thereby creating low and high pressure in much the same way.
Fixed wing aircraft can and do fly higher. I'm not sure if helicopters can safely reach the top of Mount Everest, whereas fixed wing aircraft fly over the top of it daily. I believe (and could be wrong) air density at higher altitudes will not support the weight of a helicopter without the rotors rotating faster (there is a limit to how fast they can go) to produce more lift.

Actually, a wing does end up pushing air downwards...  See https://aviation.stackexchange.com/a/21685/69

Admiral, insofar as Mount Everest, A Squirrel did just that.
On May 14th, 2005 at 7h08 (local time), a serial Ecureuil/AStar AS350 B3 piloted by the Airbus Helicopters X-test pilot Didier Delsalle, landed at 8,850 meters (29,035ft) on the top of the Mount Everest (Kingdom of Nepal).  It is also a world altitude takeoff record ... but I'll offer that it's not an every day thing.

@KorvinStarmast Well, this answer makes it sound like air is not pushed down by a wing, so a slight clarification would make this read better. Just a simple suggestion.

@Lnafziger yeah, the answer could use a bit of revision.  I have never preferred to use "the wing pushes air down" phrasing to explain lift.  But that's a matter of style, since the more neutral view I tend to take is "where is delta P taking you, up or down?" but that's getting off topic.

Most "conventional" helicopters (those with a tail rotor or fan) cannot be as efficient as a fixed wing in any flight regime.
This is because the tail rotor or fan uses a considerable amount of the power available to counter torque and to control the helicopter in yaw.  This power contributes nothing towards climbing or forward speed.
For all helicopters, the lift drag ratio is less than for fixed wing.  In order to climb to high altitudes, more lift must be generated which can only be done by increasing the pitch of the blades.  The rotor RPM is limited by two effects; a, dissymmetry of lift and b, a large increase of drag as the tips approach supersonic.  These limits means that increasing pitch is the only way to continue to climb and eventually, you will simply run out of angle of attack as drag overcomes the ability of the engine to deliver power.


text:
What does "time at temperature" mean in the maintenance manual of the J79 engine?
I was reading the maintenance manual of the General Electric J79 and saw these two charts:

and

What does "time at temperature" mean?

@mins I don't get why this manual doesn't have any glassory or a part for the parameters which are defined! I think you're right. It should not be that much complex we think. Anyway, I will wait to hear from other fellas.

The international language for aircraft maintenance documentation is English. In fact the documentation is carefully written in simple English so it is easy for non-native speakers to understand. Anyone who claims they are an "Aerospace engineer" should be able to read and understand these graphs.

@alephzero I don't agree with you. I myself is an aerospace engineer and this is the first time that see a maintenance manual. I can guess something about these new parameters but I'm not sure so I need to ask. Aerospace engineering is not just turbine engines. It's a broad field and being familiar with English cannot help that much. Based on your ideology, then all the native speaker could become a technician of jet engine by reading a manual.

It means the time the engine has been operating at the indicated Exhaust Gas Temperature (EGT).  These are limitation graphs for startup and running to prevent thermally damaging the hot section of the engine during a hot start.  It’s the chief vulnerability of a jet or other aviation gas turbine engine - they’re pretty robust and reliable, but don’t ever, ever, EVER overheat them during start up.

Sorry I'm not a native speaker but don't get this part "the engine has been operation". You meant "the engine has been operated"? and another question, it looks based on the first graph the temperature below 700 C is safe and based on the second graph, the temp. below 630 is safe. Why are they different? why not e.g. 630 for the two graphs?

Well you don't never ever want to overheat the engine in any phase of operation. Start is no different, it is easier to overheat them at start though. @Roh I edited for more comprehensible term "running". The reason for different temperatures is, that during startup short exceeding of higher temperatures is not as critical as engine parts are not heated yet. So even the exhaust is hot, the engine and its parts are not as hot yet. Also, during start there is not as much load on engine parts (it's basically at idle).

@Roh Read the labels on the graphs. The first graph is about engine startup. The second graph is about all conditions except startup. Also, note the time axis is seconds on the first graph and minutes on the second graph. The second graph also says "indicated temperatures above 750 for a maximum of three seconds are acceptable.


text:
Am I calculating the fuel consumption correctly?
I'm trying to calculate the fuel consumption for l550e engine. In its datasheet you can see this graph:

At 6500 RPM, engine power is around 48hp and the SFC is 345g/HPh. Ok, then the fuel consumption rate is:
fuel consumption rate: 345g/HPh * 48HP = 16560g/h
Am I right? We can assume the density of gasoline is 0.755g/ml based on here. Thus this engine at this RPM consumes 22Lit/h. right? Isn't it too much high?

Your math is right.
For comparison, the Rotax 582 which is also a two-stroke engine, outputs 48 kW (take-off performance) and uses 26.5 L/h. The stated specific fuel consumption is 425 g/kWh (317 g/HPh), which is close enough to the Limbach number.
Small two-stroke gasoline engines are generally less efficient than four-stroke ones.

Yes 2 strokes have an SFC roughly 25% higher than 4 strokes, more or less comparable to turboprops.


text:
Why do the intake of some helicopters have a bump body?
Not all but the intake of many helicopters have a bump body. Why? e.g.

I don't know of the Apache, but the Russian helicopters have a dust separator there. This link shows how the dust protection system works on the Mi-17. The Mi-28 and Ka-52 have the same engines, so the system probably works similarly.

The system works by making the air follow a curved path from entry to the first stage of the compressor. Due to inertia, heavier-than-air particles are forced to the outside of the curve, which is the center of the intake protrusion. Air in the separator is forced through baffles to separate the dust from the air, then through a channel that leads overboard. A bleed air nozzle at the end of the channel creates vacuum at the baffle section to aid with removing dust.
The system reduces engine performance as it reduces the compression ratio with bleed air.

Basically a cyclonic vacuum cleaner lol.

A giant, 2500 hp vacuum cleaner, yes. I want one.

Thank you for your answer but I think your answer would be better if add the details of that link here. again thank you


text:
What's for the "smoke fittings" in this muffler?
Just was reading this manaul => DLE-120 engine User Manual
In this page (pg 4), saw this part:

What's for the "smoke fittings" or "fuel tubing"in this muffler?

It looks like a fitting for an airshow smoke system

If you want smoke for this engine, here's how.  https://www.flyinggiants.com/forums/showthread.php?t=182073

It is indeed so that you can generate smoke from the exhaust, typically for airshows. The general idea is that you introduce oil or specific liquids that have been created for specific colours to the hot muffler, which will create smoke. This is how skywriting works, the pilot can turn the system on and off, like lifting their pen from the paper.
If you're not using it, it does need to be plugged as otherwise exhaust (and sound!) will exit the hole rather than the correct exit at the end of the muffler. The "fuel tubing" mentioned in the manual is simply the same tubing used for fuel supply, as the diameter is suitable for it to slot over the nodule, and for the 8-32 screw to fill the inside diameter of the tubing completely, as the diagram indicates.


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What's this part on the wing of BAE Systems Avro 146-RJ100?
It was always a question that what's this part on the wing of BAE Systems Avro 146-RJ100:

Is it kinda aerodynamic component?

Chemtrail displacement flap

They appear to be "Stall Strips" and are used to trigger a "premature" flow separation of the section of area of wing affected, that is, stall that section, before the rest of the wing gets to its natural stalling angle of attack.
Usually you see them near the wing roots but for some reason BAE saw fit to put them a little farther outboard, possibly to mitigate an excessive pitching moment if the wing was stalled at the root (due to the wing sweep which would cause the overall center of lift to shift aft when lift is removed from the root), or, possibly to the get the turbulence induced by the tripped zone away from the horizontal tail where it could cause a fair amount of vibration (they do appear to be placed to put their wake just clear of the tail surfaces).
Stall strips are one of the most "painless" of engineering band-aid mods to cure some kind of unpleasant stall behaviour, or other low speed behaviour, found during testing. At normal AOA's, the strip is in the leading edge stagnation zone (the point where the flow splits to go above or below), so there is little to no drag penalty, and all you are giving up is a bit of the potential maximum lift the wing can make, since you are taking out part of the wing's lift a bit before it normally would.
Stall strips are very effective at transforming light aircraft with very bad natural stall behaviour (sudden, without or with little warning) into very docile machines that can be flown around will full aft elevator just bobbing and shaking, all because the flow was tripped prematurely at that spot and which partially unloaded the tail, preventing further increase in AOA.  Sometimes they are just used to create the turbulence itself that impinges on the tail, just enough to create vibration that can be felt through the stick.

So just to help understand, these things actually deliberately "create" stall - is that right?

Yes.  Couple degrees AOA prior to the natural one. Where you place the strip will determine the angle.  Where they are very small (short) they're typically being used to make turbulence that will impinge on the tail and make it shake, an artificially generated pre-stall buffet.  Useful on an airplane who's wing stalls with no natural pre-stall buffet where it just kind of lets go all of the sudden.

Also.

fascinating stuff, cheers


text:
What's the starting envelope of APU?
I was reading the APS-3200 training manual (pg 268) and faced this part:

What's the starting envelope of APU?
What does mean "Normal start throughout the operating envelope: minus 300 m to 11900 m (minus 1000 ft to 39000 ft)"?

The starting envelope determines the range of altitude (and possibly temperature) at which the APU can be started in flight. In your case, the manufacturer specifies that the APU can be started at altitudes between 1000 ft below MSL and 39,000 ft above MSL, which is also the operating envelope of the APU.
Here is an APU operation and restart envelope of an Airbus A320:

(source: Airbus A320 Limitations - APU)
The cyan range is the operation and restart envelope (as a function of altitude and temperature). This APU model can only be used for bleed air and electrical power up to 22,500 ft altitude. Above, it can only provide electrical power. It also says that the battery is capable of starting the APU below 25,000 ft.

At too high an altitude, the starter motor cannot spin the APU fast enough at light off to prevent thermal damage (insufficient airflow due to low air density, due to altitude); there will come a point at which insufficient cooling airflow is available internally during the start process, during bleed air use, or for cooling of the APU generator, each of which may have different altitudes (start, electrical use, bleed use).
During the start process, the APU may experience a hot start, or failure to light off due to insufficient airflow.
The limitations in the manufacturer manual for the APU may be different than the limitations in the aircraft manual for which the APU is installed.  Ultimately, the limitations that apply are those of the aircraft manufacturer, which takes into account the APU manufacturer limitations, as well as aircraft limitations with regard to installation location, etc.